A high-speed D/A (digital to analog) converter and a high-speed A/D (analog to digital) converter have been used in communication devices at a transmission side and a reception side with an increase in speed and capacity of digital communication. With such increase in speed of the D/A converter and the A/D converter, the communication devices at the transmission side and the reception side have come to be able to perform various compensation processes and an advanced waveform equalization process by means of a DSP (Digital Signal Processing).
For example, in digital coherent optical communication, a communication device is known which performs processes such as the compensation process for frequency/phase offset, the compensation process for polarization fluctuation, and the compensation process for chromatic dispersion, or the like by a DSP. As an example, in Non Patent Literature 1, a reception device is described which performs the compensation process for chromatic dispersion to a reception signal by a DSP.
In Patent Literature 1 and Patent Literature 2, a transmission device is described which performs the compensation process for chromatic dispersion using a pre-equalization technology to signals to be transmitted by a DSP. As shown in FIG. 14, the transmission device encodes input data in an encoding circuit, and then performs in advance a waveform shaping process to cancel waveform distortion arising in a transmission line by a pre-equalization operation circuit based on an amount of chromatic dispersion of the transmission line. The transmission device outputs, using the D/A converter, an analog signal proportional to a value indicated by a digital signal output from the pre-equalization operation circuit, and drives an I/Q modulator through a driver amplifier to generate an optical transmission signal.
FIG. 15A shows an example of a transmission waveform of the optical signal transmitted from the transmission device. FIG. 15B shows an example of a reception waveform of the optical signal input into a reception device after the optical signal with the transmission waveform shown in FIG. 15A has been transmitted. Thus, it turns out that a good reception waveform with the chromatic dispersion in the transmission line compensated can be obtained by using the transmission waveform to which a pre-equalization operation has been performed.
Such chromatic dispersion compensation process by the DSP replaces a conventional chromatic dispersion compensation process using a dispersion compensating fiber. As seen above, with an increase in speed of the D/A converter and the A/D converter, it has become possible to perform the various compensation processes by means of the DSP in a transmission side or a reception side, which have been controlled by analog processes. This enables the communication device to improve and its cost to decrease, and enhances the practicality of digital coherent communications.
Non Patent Literature 2 describes an example of a D/A converter used in the transmission device in the high speed and large capacity digital communication which is represented by such digital coherent communication. The D/A converter is configured so that an output level interval for each code value of the input will be as equal as possible. As a result, the D/A converter has a linear transfer characteristic as shown in FIG. 16. In FIG. 16, when code values represented by an input waveform (a) are input into the D/A converter, the D/A converter outputs an analog signal represented by an output waveform (b). Thus, the D/A converter can perform the D/A conversion with high-resolution (effective number of bit: ENOB) and low distortion.    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2007-267001    Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 2008-124893    Non Patent Literature 1: Doug McGhan, Charles Laperle, Alexander Savchenko, Chuandong Li, Gary Mak, and Maurice O'Sullivan “5120-km RZ-DPSK Transmission Over G.652 Fiber at 10 Gb/s Without Optical Dispersion Compensation”, IEEE Photon. Technol. Lett., vol. 17, no. 3, pp. 714-716, March 2005.    Non Patent Literature 2: Yurity M. Greshchev, Daniel Pollex, Shing-Chi Wang, Marinette Besson, Philip Flemeke, Stefan Szilagyi, Jorge Aguirre, Chris Falt, Naim Ben-Hamida, Robert Gibbins, Peter Schvan, “A 56 GS/s 6 b DAC in 65 nm CMOS with 256×6 b Memory”, ISSCC Dig. Tech. Papers, pp. 544-634, February 2011.